1. Field of the Invention
The present invention relates to microlithographic projection exposure apparatuses such as are used for the production of microstructured components. The invention relates, in particular, to projection exposure apparatuses with a immersion projection lens.
2. Description of Related Art
Integrated electric circuits and other microstructured components are conventionally produced by several structured layers being applied onto a suitable substrate, which may be, for example, a silicon wafer. In order to structure the layers, the latter are firstly covered with a photoresist that is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet spectral region (DUV). Subsequently the wafer that has been coated in this way is exposed in a projection exposure apparatus. In this process, a pattern of diffracting structures contained in a mask is imaged onto the photoresist with the aid of a projection lens. Since the lateral magnification in this case is generally less than 1, projection lenses of such a type are frequently also referred to as reduction lenses.
After the photoresist has been developed, the wafer is subjected to an etching process, as a result of which the layer is structured in accordance with the pattern on the mask. The photoresist left behind is then removed from the remaining parts of the layer. This process is repeated until all the layers on the wafer have been applied.
One of the most prominent objects in the development of the projection exposure apparatuses is to be able to define lithographically structures having increasingly smaller dimensions on the wafer. Small structures result in high integration densities. This generally has a favorable effect on the performance of the microstructured components produced with the aid of apparatuses of such a type.
The size of the definable structures depends, above all, on the resolving power of the projection lens that is being used. Since the resolving power of the projection lenses is proportional to the wavelength of the projection light, one approach for the purpose of decreasing the resolving power consists in employing projection light having shorter and shorter wavelengths. The shortest wavelengths that are used at present are within the deep ultraviolet spectral region (DUV) and amount to 193 nm or occasionally even 157 nm.
Another approach for the purpose of decreasing the resolving power starts from the idea of introducing an immersion liquid having a high refractive index into an immersion interspace that remains between a final lens of the projection lens on the image side and the photoresist or another light-sensitive layer to be exposed. Projection lenses that are designed for immersion mode, and that are therefore also designated as immersion lenses, may attain numerical apertures (NA) of more than 1, for example 1.3 or 1.4. However, the immersion not only enables high numerical apertures, and thereby an increased resolving power, but also has a favourable effect on the depth of focus. The greater the depth of focus, the less demanding are the requirements as regards an exact positioning of the wafer in the image plane of the projection lens. In the broader sense, one also speaks of immersion when the light-sensitive layer is covered by an immersion liquid without the final optical element of the projection lens on the image side necessarily being immersed in the immersion liquid.
The implementation of immersion mode, however, requires considerable additional efforts in terms of structure and process engineering. For instance, it has to be ensured that the optical properties of the immersion liquid are spatially homogeneous and temporally constant—at least within the volume exposed to the projection light—even when the wafer with the photoresist applied thereon is moving relative to the projection lens. At the present time the associated technological problems have not been solved satisfactorily.